1. Field of the Invention
The present invention relates to a synchronization of a carrier frequency in an orthogonal frequency division multiplexing (OFDM) transmission apparatus, and more particularly, to a system capable of synchronizing a carrier frequency efficiently even if a carrier frequency offset is above the frequency bandwidth of one subchannel. This synchronization is accomplished by incorporating both a coarse mode for synchronizing an integer part of the carrier frequency offset and a fine mode for synchronizing a prime part of the carrier frequency offset when performing the synchronization of a carrier frequency in the OFDM transmission apparatus. The present application is based upon Korean Application No. 5786/1997, which is incorporated herein by reference.
2. Description of the Related Art
Digital televisions generally adopt a frequency division multiplexing (FDM) transmission system which uses one of the multiple carrier modulation methods. Such an FDM system occupies almost the same frequency bandwidth required for transmitting data on a single carrier because the data has a longer symbol period when transmitted in parallel on several subchannels. The FDM signal is generated by an inverse fast Fourier transform (IFFT) and are modulated in parallel by carriers which are orthogonal to each other.
Synchronization of the carrier frequency should be achieved between a transmitter and a receiver before performing a fast Fourier transform at the receiver. When the carrier frequency between a transmitter and a receiver is offset, the carrier frequency offset is divided into an integer part and a prime part through a normalization of the frequency bandwidth of one subchannel.
The integer part of the offset causes the signals generated by the fast Fourier transform (FFT) to be circularly shifted at a receiver, and the prime part of the offset causes the power and phase of the signals to be disturbed by an interference with the subchannels.
As described above, when the carrier frequency is offset between a transmitter and a receiver, the offset causes interference between the carriers of subchannels, and orthogonality between the subchannels cannot be maintained, thereby increasing the error rate of transmitted data.
In the OFDM transmission systems using methods which can be used in a terrestrial high definition television, an 8K discrete Fourier transform (DFT) and a 2K DFT are standard. The recovery of the carrier has been developed as the main technology of the transmission.
The existing methods for synchronization used in the OFDM transmission system synchronize the carrier frequency by considering that its phase is rotated uniformly when the carrier frequency is not synchronized.
The conventional methods for synchronization in the OFDM transmission apparatus work well when the carrier frequency offset is within the bandwidth of one subchannel. When the carrier frequency offset is, however, above the bandwidth of one subchannel, it is difficult to measure the rotational amount of the phase, and synchronization fails.